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The vocabulary that a term reference field is associated with is held in the data column in the field_config table. Unfortunately that's a serialised PHP array so there's no clean way to query it directly.

The Field Info API doesn't seem to have a function to query fields by type so you'll need to implement your own. The basic logic would be to grab a list of fields that have the taxonomy_term_reference type, along with their data, then loop through each to determine if it has the correct vocab assigned.